Electronic appliances of the “home gateway” or “modem” type, or more generally electronic appliances for transferring data between a telephone line and any kind of terminal such as a computer, need to be protected against lightning impacts that might occur on conductor elements of the telephone line.
In this context, ITU-T Recommendation K21 defines reference waveforms representative of lightning impacts. These waveforms are pulses having a maximum amplitude in common mode of 1.5 kilovolts (kV) for a so-called “K21 normal level” wave, or of 6.0 kV for a so-called “K21 enhanced level” wave.
Designing protection against lightning thus consists in providing means that enable the appliance to withstand one or the other of those waveforms without damage.
In practice, the design of such protection against lightning is made complicated in the context of a gateway that is designed to be installed in a home.
In order to be effective, protection against lightning requires the gateway to be connected to earth ground. Unfortunately, it is not possible to ensure that the final user will connect the appliance in effective manner to earth ground. In addition, the quality of a user's earth ground constitutes an additional unknown that also cannot be determined in advance.
A home gateway is electrically powered by a mains unit, itself connected to an electricity network of the 230 volts (V) type or of the 110 V type, and it is also connected to a digital subscriber line (DSL) typically comprising two conductor elements.
The gateway includes in particular internal components known as analog front-end (AFE) components that are coupled to the DSL line and that need to be protected against lightning.
When the gateway is not connected to earth ground or when its connection is poor, and a lightning impact occurs, then the electrical ground of the appliance may be raised to substantially the same potential as the conductor element of the DSL line, i.e. to a potential close to that of the surge voltage, which for a K21 enhanced level waveform is 6.0 kV.
In this situation, the other components of the gateway that are connected to electrical ground are subjected to a potential that is substantially equal to 6.0 kV. This applies in particular to components of the Ethernet port of such a gateway.
At present, the components of an Ethernet port are designed to withstand pulses having a maximum amplitude in common mode of 1.5 kV (K21 normal level). Thus, the 6.0 kV surge voltage that is imposed on electrical ground in the event of lightning allows the front-end components of the gateway to be protected, but nevertheless causes the Ethernet port to be damaged if the gateway is not connected to earth ground.